Originally Published MDDI October 2005
Medical device manufacturers can leverage the good dielectric properties,
lubricity, and biocompatibility of fluoropolymers to meet the demanding
requirements of their devices.
By Karl Graffte
Multilumen fluoropolymer tubing is often used in minimally invasive medical device applications.
Since their accidental discovery in the 1930s, fluoropolymers have found homes in demanding applications, ranging from the extremes of space flight to the depths of deep oil exploration. In such applications, dielectric properties, superior chemical resistance, and a broad continuous-service temperature range are critical.
In the medical device market, fluoropolymers have proven themselves in exacting applications. They have been used in devices that treat coronary angioplasty as well as in permanently implanted vascular grafts. The medical applications set forth in this article show how device manufacturers can use fluoropolymers to meet strict device requirements such as lubricity and biocompatibility.
Discovery. It all started with a failed experiment on April 6, 1938, at a DuPont laboratory in Deepwater Point, NJ. DuPont scientist Roy J. Plunkett, PhD, was working on developing gaseous Freon-based coolants. As the story goes, when he returned to his laboratory one morning, he discovered that a cylinder was showing no pressure. Curiously, though, the cylinder had lost no weight. Cutting the cylinder open, Plunkett discovered a white, waxy powder—the first polytetrafluoroethylene (PTFE). Testing later showed that this new material had properties unlike any other plastic.
Commercialization. By 1947, DuPont began to commercialize PTFE and introduced it as Teflon. Its use as a coating made Teflon a household name, and its popularity spread to a wider range of applications as new resin grades were introduced (see Table I).
Although PTFE coating technology was well developed, processing the resin into extruded or molded forms was significantly more challenging. Extruded grades of PTFE were first used in the wire and cable industry in the 1950s, where the good dielectric properties of the material proved critical to the developing electronics market. Other resins with improved processability were then developed over the next few decades.
The first Teflon tubing was made by extruding PTFE over a wire and then removing it—a labor-intensive process. The legacy of this process can be seen today in the catalogs of PTFE tubing manufacturers that list their standard products in American wire gauge (AWG) sizes. The inner diameter of the tubing is based on the outer diameter of the wire size that was originally used to make it.
In the 1960s, technology emerged that could perform the extrusion of PTFE without a wire core. This process enables PTFE tubing to be economically produced in long continuous lengths.
Also in the 1960s, the first medical device applications for fluoropolymers emerged. Advances in the vascular and coronary fields and the commercialization of several melt-processable fluoropolymers contributed to this manifestation. The 1970s saw the introduction of computed tomography scanning, coronary angioplasty, immunosuppressive drugs, and surgical lasers. These breakthroughs led researchers to look closely at fluoropolymers for device construction, owing to their inherent chemical properties and improved processability.
In the medical device market, the use of fluoropolymers centers on two key properties: lubricity and biocompatibility. Fluoropolymers exhibit very good lubricity compared with other plastics. PTFE is the most lubricious polymer available, with a coefficient of friction (COF) of 0.1, followed by fluorinated ethylene propylene (FEP), with 0.2. These two polymers represent the vast majority of all fluoropolymer tubing used in medical devices.
The biocompatibility of any polymer used in a medical device is an obvious concern. Fluoropolymers, especially PTFE, excel in this area and have a long history of in vivo use. Medical-grade fluoropolymers should meet USP Class VI and ISO 10993 testing requirements. Of course, processing cleanliness is an important factor. A polymer processing plant should have special packaging options and a cleanroom to meet device OEM's needs.
Fluoropolymers also excel in a range of other properties. Many of these properties are important in medical device manufacturing applications and vary depending on how the material is being used. Table I lists common properties of different fluoropolymers.
Table I. Common fluoropolymers and their key properties.
Chemical Makeup. The secret to the properties of fluoropolymers is rooted in their chemistry. Fluorine has the highest electronegativity of all the elements and is extremely reactive. In a fluoropolymer, the fluorine is bonded to a carbon backbone (see Figure 1). The reactivity of the fluorine protects the carbon and wraps around it as a protective sheath. As the amount of fluorine is decreased in the various polymers, the key properties are also reduced.
Figure 1. PTFE molecules have fluorine (F) bonded to a carbon backbone (C).
PTFE has the most fluorine in the fluoropolymer family and, as a result, it offers superior property ratings when compared with other polymers. In a PTFE molecule, the fluorine atoms surround the carbon backbone. By contrast, polyethylene (PE) has a similar structure, but hydrogen replaces the fluorine (see Figure 2). This not only changes the configuration of the molecules but also reduces the material's strength. As a result, PE is not suitable for many applications in which PTFE excels.
Figure 2. In PE molecules, hydrogen (H) bonds to a carbon backbone (C).
One of the most distinguishing features of fluoropolymers isn't apparent in the finished products. The processing methods required to make fluoropolymer tubing are unique and require special skills and equipment. Most are processed by melt extrusion, a method in which a plastic develops flow upon melting in normal extrusion equipment. This technique exposes the fluoropolymer to very high temperatures to lower viscosity and improve flow for long, continuous product lengths. PTFE is processed by a multistep treatment called paste extrusion, which requires customized equipment.
Melt Extrusion. The process of extruding most fluoropolymers is similar to conventional extrusion technologies. Fluoropolymers such as FEP, perfluoroalkoxy, and polyvinylidene fluoride will melt flow when heated sufficiently, typically above 500ÞF, depending on the resin type. This enables an uninterrupted feed of the resins into an extruder to produce long continuous lengths. By contrast, PTFE extrusion is limited by the size of the preform and tubing.
PTFE tubing can be extruded to very tight tolerances.
Extruding melt-processable fluoropolymers requires special equipment. Foremost are the requirements for special high-temperature extruders capable of reaching 800ÞF. The molten fluoropolymers are corrosive to many metals and, as such, require the use of nickel-based alloys for the flow paths. Such alloys exhibit excellent heat resistance, strength, corrosion resistance, and fabricability.
Paste Extrusion. The PTFE paste extrusion process was originally developed in the 1950s for extruding PTFE over wire. PTFE resin for extrusion is supplied as a fine powder that resembles flour and is extruded at room temperature. The resin must be stored in a cool room and handled with extreme care to prevent bruising. All handling should occur below the polymer's transition temperature of 19ÞC to avoid mechanical shear fibrillation of the fine particles. It is first mixed with a hydrocarbon extrusion aid or mineral spirits. During the compounding process, other fillers can be added, such as pigments, bismuth for radiopacity, glass for mechanical strength, etc. The compound is then rolled in glass jars to disperse the additives and allowed to age for a number of days before it is ready to use. The aging process ensures complete diffusion of the lubricant into the polymer particles.
Once the aging cycle is completed, the powdered resin is hydraulically compressed to make a cylindrical billet of resin called a preform. The preform is then loaded into the extruder, and a die and mandrel are clamped in place above it. As the extrusion process starts, the extruder presses the preform against the die and mandrel, forcing the resin to become compression molded into the desired shape. The tubing in this stage is referred to as green and can be easily crushed.
It then moves into a first series of ovens, where the hydrocarbon extrusion aid is vaporized out of the tube. The temperature of these ovens is set below the flash point of the extrusion aid, typically around 500ÞF. Next, the tubing moves into a second set of ovens, called sintering ovens. Here, the fine particles are heated to more than 600ÞF, causing them to become a gel and fuse together. In the sintering stage, no additional molding or support of the PTFE is required, because it does not polymerize like other plastics.
The tubing can then be run through a series of in-line processes such as chemical etching, cutting, and others. It can also undergo postextrusion expansion to make heat-shrink tubing.
Because PTFE is not melt-processable like other polymers, the tubing can be manufactured to tighter tolerances. PTFE tubing can be made with wall thicknesses as little as 0.001 in. and tolerances of ±0.0005 in. This is due in large part to the fact that the PTFE will not melt flow, so it can be controlled more precisely. An example of a product requiring tight tolerances is multilumen tubing with multiple precision passages.
Depending on the dimensions of the tubing, extruders of various sizes and configurations can achieve the desired results. PTFE is commonly extruded in vertical and horizontal processes. Some of the largest PTFE extruders require three or four floors of a manufacturing plant.
The biggest drawback of the PTFE extrusion process is that the dimensions of the tube and the size of the preform limit the length of the extrusion. Once the preform billet is expended, the extruder must be set up again for the next run.
Working with Fluoropolymers
Fluoropolymers require some special considerations when using them to create medical tubing. It is difficult to bond a highly lubricious material, and thus this is a primary challenge for design engineers.
Etching. As the saying goes, “If nothing sticks to Teflon, how does Teflon stick to the pan?” In the case of a frying pan, it's a mechanical attachment of the Teflon coating to the rough surface of the metal. But for a thin-wall extruded tube used in a medical application, the solution is a chemical bond facilitated by etching the surface.
A sodium napthalene solution can remove a fraction of the fluorine atoms from the surface of the tube. The fluorine is then replaced with bondable molecules that are more amenable to adhesion. The etching process occurs at a depth of only a few angstroms and does not significantly affect the mechanical properties of the tube.
The process of etching is most commonly performed along the outer diameter of tubing used as a lubricious liner in catheters. Site-specific etching, most commonly on the tip of a catheter, can also be accomplished.
Flaring and Flanging. Other forming operations, such as flaring and flanging, can be performed using techniques also employed with other polymers. The primary difference is that higher temperatures (above 500ÞF) are required to melt the fluoropolymers. This often requires custom equipment and temperature-resistant nickel-alloy parts.
Other fluoropolymers can be used as tools for these operations. Because of its high-temperature properties, FEP can be used as a heat-shrink fusing sleeve to join lower-temperature polymers. An example of this is the butt-welding of polymers in the nylon family (i.e., Pebax) in catheter construction, described below.
The use of fluoropolymers in medical applications is so extensive that a detailed exploration of the market is beyond the scope of this article. However, there are a few well-established applications that highlight the key properties of fluoropolymers (see Table II).
Table II. Medical applications in which fluoropolymers are commonly used.
Guiding Catheter. Used to deliver coronary stents and other devices, the guiding catheter has a well-established history in the medical device market. At the core of most guiding catheters is a PTFE inner liner. The superior lubricity of PTFE has made it the material of choice for this application, with the lowest dynamic COF of any polymer. ASTM D1894 defines COF standards. In effect, the lower the COF, the more lubricious the material. Lubricity is so critical that even FEP (the second most lubricious material available) has not proven successful as a catheter liner. During the construction of a guiding catheter, PTFE is chemically etched onto the tube's outer diameter. This process allows for the bonding of materials to the outer diameter of the liner. The bonding is accomplished by using an FEP heat-shrinkable fusing sleeve. The high shrink temperature of the FEP (350ÞF) is above the melting point of the Pebax inner layers. The FEP provides sufficient constrictive hoop strength to fuse the polymers together as the heat shrink recovers. The FEP can then be removed from the device, leaving a smooth outer jacket.
PTFE Introducer. Developed and patented in the 1970s by Cook Inc., the PTFE introducer utilizes a little-known property of PTFE. PTFE can be processed in a manner that allows for molecular orientation of the material. In such cases, the PTFE tube can be split and torn longitudinally. The tubing must first be scored precisely at the edges, enabling the tear to be easily facilitated by hand along the longitudinal grain of the molecules. The effect is similar to a tear notch on a food package. During use, a surgeon can remove a PTFE introducer from a patient while the primary device remains in place.
Multilumen Catheter. Fluoropolymer multilumen tubing has found a home in minimally invasive medical devices. The multiple passages of today's fluoropolymer catheters enable surgeons to perform a series of procedures without the need to remove one catheter and insert another. The demands of the medical device community will continue to expand as it is challenged to reach previously inaccessible areas of the vasculature. Because of their good lubricity, biocompatibility, and chemical resistance properties, fluoropolymers will fit the bill for these devices.
Extruding melt-processable fluoropolymers requires special high-temperature extruders that can reach 800ÞF.
The unique properties of fluoropolymers have made them the material of choice in the design of many minimally invasive medical devices. The properties of these materials give design engineers the latitude they require in developing and commercializing next-generation devices. With a history going back to the first minimally invasive medical devices, fluoropolymers have earned their reputation as the workhorse of the industry.
The continued development of fluoropolymer extrusion technology has enabled medical device engineers to further refine catheter technology while developing new and innovative surgical devices. Neurological catheters are one example of this refinement. Precision microminature extrusions, such as very-thin-wall tubing, can be used in such devices because of their ultrathin walls and precise tolerances. Another example is multilumen tubing. Its multiple passages allow surgeons to conduct multiple procedures using the same catheter.
Refining tolerances and extrusion capabilities are important because catheter designers are concerned with tolerance stacking, where the production of materials on the high side of the tolerance range can reduce the workable inner diameter of the catheter.
New materials also present interesting opportunities for advances in catheter designs. To a certain degree, the far extremes of production capabilities are based upon the materials that are currently available. However, new materials will allow polymer processors to continue to refine their capabilities.
The continuous innovation of new polymer blends and extrusion technologies make these materials the ideal starting point for innovative designs.
Copyright ©2005 Medical Device & Diagnostic Industry